It is known in the art that activated carbon can remove undesired odors, noxious fumes, organic vapors and the like from air and fluid streams. It is also known to use these activated carbon granules in a variety of structures including packed beds, loaded nonwovens, loaded foams, and bonded adsorbents. Granular adsorbents have been bonded and molded into shapes suitable for filtration of gases.
For example, Breger et al. U.S. Pat. No. 3,217,715, and Tobias, U.S. Pat. No. 3,474,600, propose extruding cylindrical, or other shaped, pellets of activated carbon which pellets are then placed into a supporting structure. The rods exemplified were an extruded mix of activated carbon and polyethylene binder. The purpose of these patents was to provide improved pressure drops over carbon bonded into a monolithic structure (e.g., a sheet). U.K. Patent No. 1,390,668 stated that a drawback with this extrusion process was that during extrusion a substantial portion of the pores within the rods became clogged resulting in a decrease in adsorption capacity and an increase in pressure drop when the extruded rods were used alone (i.e., a cigarette filter), rather than a packed bed. This patent proposed forming sheets of bonded activated carbon using a polyolefin binder by mixing hot carbon with the binder, collecting the mixture on a belt, leveling the mixture, and compressing the mixture. The objective is to eliminate shear forces in forming the bonded carbon body.
U.S. Pat. No. 3,538,020, Hassett et al., proposes forming filters by coating adsorbent particles with a liquid prepolymer binder, placing the coated particles in a mold and curing. A cartridge formed by this method allegedly had three combined advantages not simultaneously possessed by any of the tested commercial cartridges, low cost, good efficiency and low pressure drop. A similar technique is discussed in U.S. Pat. Nos. 3,544,507 (Lloyd) and 3,721,072 (Clapham) who used a binder emulsion or solution of a vinylidine polymer and a water soluble thermosetting aminoplast to coat activated carbon. The object of the Lloyd patent was to prevent dusting and provide adequate binding of the activated carbon particles without a great loss in adsorption capacity. Clapham describes forming such coated carbon granules into a pleated structure to provide improved pressure drop performance.
U.S. Pat. No. 4,061,807 (Shaler et al.) describes a method where water is used to promote adhesion between a binder and a granular adsorbent material prior to molding the binder and adsorbent and heating to consolidate the material into a self-supporting filter. This allegedly provides a simple method to manufacture a self-supporting adsorbent filter.
Reduction in pressure drop is a concern with U.S. Pat. No. 4,981,501 (Von Blucher et al.). This patent proposes dipping a 3-dimensional open framework into a binder or adhesive, then dipping the binder-coated framework into a mass of loose absorbent granules (about 0.1-1 mm diameter).
U.S. Pat. No. 3,919,369 (Holden) molds a self-supporting filter using a method similar to that of Shaler et al. with holes formed into the molded shape to decrease pressure drop. These open structures, however, will be deficient in efficiency.
U.S. Pat. No. 5,033,465 (Braun) discloses forming a self-supporting panel-shaped filter. The binder used is predominately of a particle size greater than 400 mesh. The patent emphasizes uniform mixing of binder and adsorbent granules and avoiding agglomeration.
U.S. Pat. No. 4,665,050 (Degen et al.) describes a self-supporting structure comprising an inorganic sorbent such as alumina, and a binder. The process for immobilizing the sorbent particles in the self-supporting structure comprises the steps of: (a) preheating the inorganic sorbent particles to an elevated temperature sufficient to slightly soften the binder particles; (b) mixing the sorbent particles with a binding material to coat the sorbent particles while avoiding adhesion between binder particles (by use of cooling or of an antiagglomerating agent); and (c) heating the mixture to about the solid-liquid transition temperature of the polymeric binding material with or without pressure to form a structure which upon cooling is self-supporting.
The above patents propose a variety of methods for employing adsorbent particles in a variety of filter applications and providing varying levels of efficiency, capacity, pressure drop and cost. The present invention is directed at forming a self-supporting filter structure having good efficiencies and adsorption capacities while providing low pressure drops. Particularly, the invention is directed at providing an adsorbent self-supporting filter for use in applications such as a recirculating filter, room filter or automotive filter where low pressure drop and good adsorption efficiency is required.